Conventional process for producing teas includes: plucking, withering, rolling, and panning. After that, teas are dried at a relatively high temperature, and the moisture content of teas is reduced to about 4% dry base moisture content. Consequently, teas can be stored for a long period without impairing their quality.
In such a drying process, air is generally heated by burning oil or gas, or by some electrical heating device. Hot air, between about 90.degree. C. to 150.degree. C., is guided to the surface of tea leaves. Then the moisture of the tea leaves is removed, and the teas are dried for storage.
The flavors of teas are produced during the withering process. Once the withering process is completed, the flavors dissipate under high temperature in the later drying processes. These flavors include polyphenol, aldehyde, ether, pyrabine, etc. Thus, the high temperature drying process is the major cause of volatile flavors loss. From another point of view: the flavors of teas when we are drinking are not as rich as the flavors of those teas in the production process.
In order to reduce the loss of volatile flavors, another process for producing "frozen fresh tea" has been used. In this process, the half-product of tea (after rolling) is not dried under high temperature. Instead, it is stored in refrigerators when its moisture content is high (at about 60 to 130% dry base moisture content). Under a relative low temperature (below 15.degree. C.), the residual enzyme is inhibited, and the teas are kept fresh so as to preserve their qualities and flavors.
Because there is no high temperature drying process, some volatile flavors are retained in the "frozen fresh tea". These flavors spread out while steeping the tea, which impress drinkers with exotic olfactory experiences. However, such progress in retaining flavor quality results at great cost from keeping the teas in refrigerators. If they are not stored in refrigerators, without low temperature inhibition the residual enzyme in the teas will rapidly react to impair the qualities of the teas under a room temperature and a relatively high moisture content.
It is very clear that although the "frozen fresh tea" preserves more volatile flavors and a constant quality, it requires refrigeration and the storage cost is significantly increased.
In addition to the aforementioned flavor keeping and storage problems, except for temperature atmospheric conditions (including moisture) are not controllable in the conventional tea producing process. Air can be heated in order to reduce the relative humidity. Among all factors, atmospheric humidity is most important. In a moist atmosphere (high relative humidity), tea leaves need more time in contact with the hot air to dry, resulting in the flavors loss. For this reason, in some regions where atmosphere humidity varies over a larger range, whether or not tea-makers produce teas depends on the weather.